This invention relates to resonant circuit structure and more particularly to resonant circuits such as relatively high frequency oscillators and other circuit arrangements wherein stray reactance and spurious resonance are important design constraints that can seriously limit circuit operation and performance.
As is well-known to those skilled in the art, the performance of resonant electrical structure is often affected by stray reactance terms such as, for example, capacitance and inductance that is attributable to the circuit interconnections or wiring and undesired reactance terms that are associated with the circuit components utilized, e.g., the parasitic or stray capacitance of an inductive element being used within the resonant circuit. Although such stray reactance sometimes can be incorporated into the circuit design or can be minimized by careful circuit layout and by judiciously selecting a type of capacitor, inductor or resistor that is most appropriate to the frequencies of interest, many situations arise wherein stray reactance remains appreciably large relative to the design values of the reactances utilized in the resonant circuit. Under such circumstances the resonant frequency of the circuit can be substantially below the theoretical design value (i.e., that which would result if ideal inductors and capacitors were employed).
The drawbacks and disadvantages that result from stray reactance terms are especially significant in the design and realization of circuitry employing tunable resonators that are intended to operate over a relatively wide frequency range. For example, the hereinafter disclosed embodiment of the invention is an electronically tubable oscillator which operates in the VHF-UHF region of the frequency spectrum and is continuously tunable over at least a full octave. Utilizing conventional circuit analysis techniques and assuming the use of variable capacitors such as state of the art varactor diodes and fixed value inductors, it can be shown that the tuning ratio of an LC resonator (i.e., the ratio of the maximum and minimum resonant frequencies) is [(C.sub.s +KC.sub.0)/(C.sub.s +C.sub.0)].sup.1/2, where the varactor diode is tunable between a capacitance value of C.sub.0 and KC.sub.0 (i.e., K is a positive real scale factor), C.sub.s represents stray capacitance associated with the resonant circuit inductor and it is assumed that the oscillator active elements and associated bias components do not affect operation of the resonator. Examining this expression for the tuning ratio it can be recognized that although K=4 will provide an octave-wide tuning range when C.sub.s =0, K must be equal to 5.5 in order to provide that same bandwidth when C.sub.s =C.sub.0 /2, (i.e., when the stray capacitance associated with the circuit is one-half the minimum capacitance of the varactor diode tuning element). Few state of the art electronically controllable devices such as varactor diodes provide the capacitance variations necessary to permit tuning over a frequency range of one octave or more when stray capacitance is considered, especially at frequencies within the UHF portion of the spectrum where relatively low values of inductance and capacitance are utilized in the resonant circuit.
Accordingly, it is an object of this invention to provide resonant circuit structure that is arranged to minimize the effect of stray reactance terms.
It is a further and more specific object of this invention to provide a tunable LC resonant circuit wherein the effect of stray capacitance is minimized to thereby optimize the usable tuning range of such resonant circuit.
It is still another object of this invention to provide an electronically-tuned oscillator that can be used over at least one octave of the VHF-UHF region of the frequency spectrum wherein the circuit is arranged to minimize stray capacitance that would otherwise reduce the usable tuning range of the oscillator.